Electron beam lithography systems of the high beam throughput type employ beam spots which are shaped either as fixed squares, circles, or some other shape. The dimensions of spots generated by the focused electron beam in the target plane (i.e., on a wafer, mask, reticle, etc.) must meet certain specifications within very close tolerances. The measurement and control of the spotsize are, therefore, very important. Unavoidable optical aberrations and electron-to-electron interaction cause edges of the beam spot to be somewhat less than sharply defined; rather, the beam current density drops off from its maximum value within the spot to zero with a finite slope and within a finite distance. Referring to FIG. 1 of the accompanying drawings, in width of this drop-off distance determines the edgewidth which is sometimes defined as the portion of the beam spot width existing between the beam current density points at ten percent and ninety percent of the maximum beam current density. More often, the twelve percent and eighty-eight percent points are employed in order to correspond to definitions established in connection with Gaussian bell-shaped aberration disks. In either case, the edgewidth determines the electron-optical resolution of the spot.
It is important to measure spotsize and edgewidth accurately and quickly in order to utilize these data to effect corrective action, such as refocusing of the beam, rotationally correcting the spot image of the deflection, adjusting the coefficients for the correction of astigmatism, etc. It is common practice to obtain these data by scanning the beam spot in the target plane across the sharp edge of a member which partially blocks the beam receiving surface of a beam current detector. The member is disposed in the target plane with the sharp edge extending perpendicular to the direction of scan with a length considerably greater than the corresponding beam spot dimension. The resulting beam current detected by the beam current detector is displayed on an oscilloscope. The spot profile in the scan direction is obtained by differentiating the beam current signal as a function of scan displacement. The differentiated signal lends itself to a graphical determination of the spotsize and edgewidth. The differentiated signal, however, has a low signal-to-noise ratio because differentiation inherently includes high-pass filtering.